Fixed the bug in drawHorizontal and drawVertical method. Which was to write...

Fixed the bug in drawHorizontal and drawVertical method. Which was to write code that would throw an exception when the user tries to draw a line on top of an existing line.

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -150,6 +150,9 @@ public class DotsAndBoxesGrid {
         }
 
         // FIXME: You need to throw an exception if the line was already drawn.
+        if (getHorizontal(x, y)) {
+            throw new IllegalArgumentException(String.format("line at coordinate (%d, %d) already exists. Try other coordinates", x, y));
+        }
 
         this.horizontals[x][y] = true; // otherwise set the horizontal line to true
 
@@ -182,6 +185,9 @@ public class DotsAndBoxesGrid {
         
         // FIX this too, whether vertical line was already drawn or not
         // You need to throw an exception if the line was already drawn.
+        if (getVertical(x, y)) {
+            throw new IllegalArgumentException(String.format("line at coordinate (%d, %d) already exists. Try other coordinates", x, y));
+        }
 
         this.verticals[x][y] = true; // otherwise set the vertical line to true